Organic light emitting diode (OLED) displays have gained significant interest recently in display applications in view of their faster response times, larger viewing angles, higher contrast, lighter weight, lower power, amenability to flexible substrates, as compared to liquid crystal displays (LCDs). Despite the OLED's demonstrated superiority over the LCD, there still remain several challenging issues related to encapsulation and lifetime, yield, color efficiency, and drive electronics, all of which are receiving considerable attention.
Although passive matrix addressed OLED displays are already in the marketplace, they do not support the resolution needed in the next generation displays, since high information content (HIC) formats are only possible with the active matrix addressing scheme.
Active matrix addressing involves a layer of backplane electronics, based on thin film transistors (TFTs) fabricated using amorphous silicon (a-Si:H), polycrystalline silicon (poly-Si), or polymer technologies, to provide the bias voltage and drive current needed in each OLED based pixel. Here, the voltage on each pixel is lower and the current throughout the entire frame period is a low constant value, thus avoiding the excessive peak driving and leakage currents associated with passive matrix addressing. This in turn increases the lifetime of the OLED.
In active matrix OLED (AMOLED) displays, it is important to ensure that the aperture ratio or fill factor (defined as the ratio of light emitting display area to the total pixel area) should be high enough to ensure display quality.
Conventional AMOLED displays are based on light emission through an aperture on the glass substrate where the backplane electronics is integrated. Increasing the on-pixel density of TFT integration for stable drive current reduces the size of the aperture. The same happens when pixel sizes are scaled down. One solution to having an aperture ratio that is invariant on scaling or on-pixel integration density is to vertically stack the OLED layer on the backplane electronics, along with a transparent top electrode as shown in FIG. 2. In FIG. 2, reference numerals S and D denote a source and a drain, respectively. This implies a continuous back electrode over the OLED pixel.
However, this continuous back electrode can give rise to parasitic capacitance, whose effects become significant when the electrode runs over the switching and other TFTs. The presence of the back electrode can induce a parasitic channel in TFTs giving rise to high leakage current. The leakage current is the current that flows between source and drain of the TFT when the gate of the TFT is in its OFF state.